Method of providing security for transmitting a digital medical image

ABSTRACT

A method of preparing a digital medical image for secure transmission, the method comprising embedding data into the digital medical image using a reversible watermarking process, generating a code for tamper detection and localization from the digital medical image using a computational function, and embedding the code for tamper detection and localization into the digital medical image using the reversible watermarking process; and a method of reviewing a digital medical image prepared by the method of preparing, the method of reviewing comprising retrieving the code for tamper detection and localization from the digital medical image; reversing the watermarking processes to obtain a restored image; generating a code from the restored image using the computational function; and comparing the retrieved code for tamper detection and localization with the code generated from the restored image to detect and locate tampering.

TECHNICAL FIELD

This invention relates to security for digital medical imagetransmission.

BACKGROUND

Hospitals routinely transmit medical images only within their internalnetwork which is protected by their firewall. However, with the adventof tele-radiology, there is an increasing need for doctors to transmitimages to healthcare professionals all over the globe to seek highquality diagnoses or second opinions. As a result, medical imagesecurity has become an important issue when medical images are beingtransmitted over open network, where sensitive patient information isexposed to hackers or individuals with malicious intents. Possiblesecurity breaches may include tampering of images to include false datawhich may lead to wrong diagnosis and treatment.

There are several mandates and guidelines in place to protect sensitivepatient information. The Health Insurance Portability and AccountabilityAct (HIPAA) requires healthcare providers to take measures to ensure thesecurity of medical images so as to protect patient's privacy. TheDigital Imaging and Communication in Medicine (DICOM) standard a s todefine a technical framework for application entities involved in theexchange of medical data to adhere to a set of security profiles. Atpresent the DICOM standard does not address the security of patient dataafter it has been decrypted, and when it is no longer under theprotection the private network.

Current security measures have their limitations. Cryptography is ableto ensure security in terms of storage and transmission but oncedecrypted the information is no longer protected. Firewalls andaccess-control methods only protect the images up to the point of theinternal networks. Authenticity problems are often a result of humanactions such as illegal distribution or human error in transmitting tounauthorized individual. To ensure the authenticity of the images, thetwo common tools used are digital signature and watermark.

A digital signature is the non-repudiation, encrypted version of themessage digest extracted from the data to prove integrity andoriginality. The security of digital signature often depends on thestrength of the hash functions used to validate the signatures. It hasbeen demonstrated that it is possible to generate two datasets withdifferent content but having the same MD5 (Message-Digest algorithm 5)hash. As a result, it is then possible to append arbitrary data to thedataset and their hash value may still be the same. In mathematicalterms, if MD5(x)=MD5(y), then MD5(x+q)=MD5(y+q)⁶ (where x and y couldrepresent two different 128 bytes dataset and q is an arbitrary datasetof any length). We can then apply these concepts to medical images, forexample, by modifying the first 1024 bits of the pixel values of animage. Consequently, two images can be nearly identical except for sixpixels and the two images can produce the same MD5 hash. This shows thatit could be possible for a hacker to tamper an image to includeartifacts that may lead to wrong medical diagnosis, while keeping theMD5 of the image unchanged. This type of tampering may also give rise toserious security issues if the image was used in a legal or policeinvestigation.

Watermarking is the practice of imperceptibly adding hidden data to thecover-signal (e.g. image, audio, video, or other work of media) in orderto convey the hidden data. In the context of medical images, the hiddendata can be used to verify the authenticity of the images. This providesan alternative technique to protect medical images. It allows messagesto be indiscernibly embedded into an image by modifying the pixelvalues. Watermarks may be permanent or reversible. Permanent watermarkspermanently modify the image in some controlled ways, while reversiblewatermarks allow these modifications to be completely reversedsubsequently by an authorized person.

Because digital medical images can be easily modified, there is also aneed to identify whether tampering has been performed on the imagesduring transmission, and to locate the regions that have been tamperedwith.

SUMMARY

This application describes a method of securely transmitting digitalmedical images comprising a secure and fully reversible watermarkingscheme which is capable of verifying authenticity and integrity of DICOMimages. The reversible watermarking utilizes a secret random locationsignal which is encrypted using public-key for security. A tamperingdetection and localization function is incorporated using a dual layerwatermarking technique.

According to a first exemplary aspect, there is provided a method ofpreparing a digital medical image for secure transmission, the methodcomprising embedding data into the digital medical image using areversible watermarking process; generating a code for tamper detectionand localization from the digital medical image using a computationalfunction; and embedding the code for tamper detection and localizationinto the digital medical image using the reversible watermarkingprocess.

Embedding the metadata using the reversible watermark process maycomprise dividing the digital medical image into non-overlapping pixelblocks, generating a random location signal designating one pixel ofeach non-overlapping pixel block as an estimator pixel, and embeddingthe metadata into one or more of the non-overlapping blocks as required.

The method may further comprise encrypting the random location signalusing public key cryptography.

The method may further comprise embedding a digital envelope into thedigital medical image after embedding the metadata, the digital envelopecomprising a concatenation of a bit stream of the encrypted randomlocation signal, a cyclic redundancy code computed for the randomlocation signal and a hash of the digital medical image.

Generating the code for tamper detection and localization from thedigital medical image may comprise dividing the digital medical imageinto non-overlapping pixel blocks and computing a cyclic redundancy codefor each non-overlapping pixel block. Embedding the code for tamperdetection and localization using the reversible watermark process maycomprise embedding each cyclic redundancy code into the non-overlappingpixel block for which the cyclic redundancy code was computed.

According to a second exemplary aspect, there is provided a method ofreviewing a digital medical image prepared by the method of the firstaspect, the method of the second aspect comprising retrieving the codefor tamper detection and localization from the digital medical image;reversing the watermarking processes to obtain a restored image;generating a code from the restored image using the computationalfunction; and comparing the retrieved code for tamper detection andlocalization with the code generated from the restored image to detectand locate tampering.

Generating the code from the restored image may comprise dividing therestored image into non-overlapping pixel blocks and computing a cyclicredundancy code for each non-overlapping pixel block of the restoredimage.

According to a third exemplary aspect, there is provided a method ofsecurely transmitting a digital medical image, the method comprisingpreparing the digital medical image using the method of the firstaspect; transmitting the prepared digital medical image; and reviewingthe prepared digital medical image using the method of the secondaspect.

According to a fourth exemplary aspect, there is provided a digitalmedical image prepared for secure transmission using the method of thefirst aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be fully understood and readilyput into practical effect, an embodiment of the invention will now bedescribed by way of non-limitative example, the description being withreference to the accompanying illustrative drawings, in which:

FIG. 1( a) is a schematic illustration of embedding data such as amessage;

FIG. 1( b) is a schematic illustration of extracting the embedded dataof FIG. 1( a);

FIG. 2 is a schematic illustration of block-by-block CRC embedding;

FIG. 3 is a schematic illustration of two layers of watermarking;

FIG. 4( a) is a Computed Tomography (CT) image;

FIG. 4( b) is the CT image of FIG. 4( a) after watermarking;

FIG. 4( c) is an X-Ray Angiography (XA) image;

FIG. 4( d) is the XA image of FIG. 4( c) after watermarking;

FIG. 4( e) is an Ultrasound (US) image;

FIG. 4( f) is the US image of FIG. 4( u) after watermarking;

FIG. 5( a) is a mammogram image;

FIG. 5( b) is the mammogram image of FIG. 5( a) after tampering to addtumor-like features;

FIG. 5( c) is the mammogram image of FIG. 5( b) displaying localizationof the tampering;

FIG. 5( d) is an X-Ray image;

FIG. 5( e) is the X-Ray image of FIG. 5( d) after tampering to addfeatures that indicate lung infection;

FIG. 5( f) is the X-Ray image of FIG. 5( e) displaying localization ofthe tampering;

FIG. 6( a) is an XA image with 1 pixel of tampering;

FIG. 6( b) is the XA image of FIG. 6( a) displaying localization of thetampering;

FIG. 6( c) is an XA image with an 8×8 pixel block of tampering;

FIG. 6( d) is the XA image of FIG. 6( c) displaying localization of thetampering;

FIG. 6( e) is an XA image with multiple 8×8 pixel blocks of tampering;

FIG. 6( f) is the XA image of FIG. 6( e) displaying localization of thetampering;

FIG. 7( a) is a CT image with 1 pixel of tampering;

FIG. 7( b) is the CT image of FIG. 7( a) displaying localization of thetampering;

FIG. 7( c) is a CT image with an 8×8 pixel block of tampering;

FIG. 7( d) is the CT image of FIG. 7( c) displaying localization of thetampering;

FIG. 7( e) is a CT image with multiple 8×8 pixel blocks of tampering;

FIG. 7( f) is the CT image of FIG. 7( e) displaying localization of thetampering;

FIG. 8 is a flowchart of a method of preparing a digital medical imagefor secure transmission; and

FIG. 9 is a flowchart of reviewing a digital medical image prepared bythe method of FIG. 8.

DETAILED DESCRIPTION

Exemplary embodiments of methods of preparing and reviewing a digitalmedical image 10 will be described with reference to FIGS. 1 to 9.

When preparing the digital medical image 10 for secure transmission,before watermarking the digital medical image 10, it is preferable topreprocess the digital medical image 10. To do so, underflow andoverflow conditions are taken care of to ensure that the selecteddigital medical image is suitable for watermarking.

Before the digital medical image 10 is watermarked, the image depth hasto be taken into account. For a digital medical image of p bits depth,there will be 2^(P)−1 possible gray levels. Occurrence of an underflowor overflow condition implies that pixel range of the digital medicalimage has been exceeded.

An underflow will occur if an intended pixel to be watermarked has apixel of gray value equal to 0. Consequently, subtracting one gray levelfrom this pixel will result in a negative value.

An overflow will take place if the intended pixel to be watermarked hasa pixel of gray value equal to the maximum allowable pixel value of2^(P)−1, for example, 255 for an 8-bit grayscale image. Hence, addingone gray value to the pixel will exceed the maximum value for a p-bitimage. As a result, pixels that have pixel gray values 0 or 2^(P)−1 arenot modifiable. DICOM images are generally stored using 16-bits perpixel and imaging modalities usually do not produce images that utilizethe full range of pixel values. Thus, in an exemplary embodiment, allimage pixels are shifted up by four pixel values. This will be describedin more detail below. After transmission of the digital medical image,upon receipt of the digital medical image, the gray levels of thedigital medical image are restored to their original values bysubtracting all the pixels by four after dewatermarking.

When preparing the digital medical image 10 for secure transmission asshown in FIG. 8, data is preferably embedded into the digital medicalimage using a reversible watermarking process 82. The embedding processseeks to protect data or source information such as patient and imagemetadata by watermarking it into the image using a random locationsignal. In this way, there is no need to transmit the metadata togetherwith the image since it is embedded into the image. This ensures that ahacker would not be able to easily separate the image header, delete itand create a new one.

The digital medical image 10 is first divided into 2×2 non-overlappingblocks 12 of 2×2 pixels 14, as shown in FIG. 1 (a). Considering eachblock 12 of 2×2 pixels, data comprising a binary message (msg) isembedded according to the following steps:

-   1. Generate a random location signal that denotes or designates one    of the four possible pixel positions 121, 122, 123, 124 in a 2×2    non-overlapping block 12 where an estimator pixel e is to reside.    Use of the random location signal is to ensure that it will be more    difficult to decipher which of the four pixel 121, 122, 123, 124 is    used as the estimator pixel e. Only one pixel (121 as shown) is    designated as the estimator pixel e, 121 in each 2×2 non-overlapping    block 12. Referring to FIG. 1( a), a, b and c represent pixels 122,    123, 124 in the 2×2 block 12.-   2. Select one pixel of the block, e.g. pixel a, 122, and compare it    with the estimator pixel e, 121.-   3. If it is satisfied that |estimator−a|<2, then pixel a, 122, is    able to carry one bit and is modified as follows:    -   a. If msg(i)=1 then a is changed to a_(w)=a+2    -   b. If msg(i)=0 then a is changed to a_(w)=a−2

A difference of 2 is used in order to increase embedding capacity.

-   4. If it is not satisfied that |estimator−a|<2, then a difference    between the estimator and a is increased by 2 by changing a.-   5. Steps 4 to 6 are repeated with remaining pixels b and c.-   6. Steps 1 to 5 are repeated until all the data or message bits have    been embedded or all the non-overlapping blocks 12 had been    processed, as required depending on length of the data or message.    In this way, a prepared digital medical image 20 as shown in FIG. 1(    b) is obtained.

For data extraction, the prepared digital medical image 20 is dividedinto the same 2×2 non-overlapping blocks 22 of 2×2 pixels 24 each, asshown in FIG. 1( b). The location of the estimator pixel e, 221 is knownusing the random location signal. Referring to FIG. 1( b), a_(w), b_(w)and c_(w) represent pixels 222, 223, 224 in the 2×2 block 22 in theprepared digital medical image 20. Using the estimator, the pixels 221,222, 223, 224 can be restored and the hidden or embedded data or messageis extracted according to the following steps:

-   1. If a_(w)> estimator then change a_(w) to a_(r)=a_(w)−2-   2. If a_(w)< estimator then change a_(w) to a_(r)=a_(w)+2-   3. The decoder will consider that a bit “1” was embedded if    |a_(r)−estimator|<2, and vice versa.

All the pixels 24 will be increased by 4 pixel values to avoid underflowbecause pixels which are allowed to be modified will be changed by ±2and this value is increased by a factor of 2 with dual layerwatermarking (described in further detail below). Hence, to avoidoverflow, the maximum pixel value allowable for an image to bewatermarked is calculated by equation (2) below:Maximum pixel limit=2^(p)−1−q−r  (2)where p is the bits depth of the image, q is the increase in all pixelvalues (i.e. 4) and r is the pixel values allowed for modification (i.e.2×2). Hence, this method supports 16-bit images with maximum pixelvalues of 65527.

The security of this method depends on the ability to keep the estimatorlocation 121 secret. Hence, in order to keep the random location signalsecure, a cryptography system known as public-key cryptography orasymmetric cryptography is used to encrypt the random location signal.The public-key cryptography makes use of a pair of codes (also known asthe public and private key) to encrypt a message. The signal which isencrypted using the public key can only be decrypted using thecorresponding private key. The main advantage of using the public-keycryptography is that the public key and the private key aremathematically related but it is computationally infeasible to deduceone key from the other. In the present method, the random locationsignal is encrypted using an RSA cryptosystem which bases security onthe difficulty of factoring large integers.

In practice, in order for a radiologist (e.g. the sender) to send animage to a doctor (e.g. the recipient) in another hospital, he wouldencrypt the random location signal with the doctor's public key (whichis widely distributed). Upon receiving the image, the doctor can onlyretrieve the embedded data by decrypting the random location signalusing his private key, which is kept secret.

In addition to embedding data such as metadata into the digital medicalimage 10, a digital envelope (DE) is preferably also be embedded intothe digital medical image 10 after the last bit of metadata has beenembedded. The DE is created by concatenating a bit stream of theencrypted random location signal, a cyclic redundancy code (CRC)computed for the random location signal and a hash of the digitalmedical image. The hash is preferably obtained using a Secure HashAlgorithm (SHA)-256. The CRC code of the random location signal iscomputed to serve as a check to ensure that the decrypted randomlocation signal is correct. A standard CRC-32 polynomial used in theIEEE 802.3 (Ethernet) may be employed to compute the CRC. The SHA-256hash code of the digital medical image 10 is calculated so that it canbe used to verify the success of dewatermarking when the prepareddigital medical image 20 has been received and is being reviewed by arecipient.

The method also comprises generating a code for tamper detection andlocalization from the digital medical image 10 using a computationalfunction 84, and embedding the code for tamper detection andlocalization into the digital medical image 10 using the reversiblewatermarking process 86.

Tamper detection and localization is useful because integrity controlbased on an exact preservation of all parts of the digital medical imagemaybe unnecessarily strict as distortions on the image may also be dueto noise originating from the transmission process. Tamper localizationwill avoid unnecessary requests for retransmission of the digitalmedical image 10 since it follows that if the tampered area is notwithin a region of interest, the image may still be consideredacceptable by a recipient. Retransmission is undesirable as it mayincrease delay time and slow down a hospital's network.

In the event that areas in the region of interest have indeed beenaltered, for example by a hacker for malicious intents, the tamperdetection and localization function would be able to detect suchalterations. Thus, the recipient of the prepared digital medical image20 can be alerted that an attack had been carried out on the hospitalinformation system.

FIG. 2 illustrates the tampering detection and localization approachimplemented in the method. The digital medical image 10 is first dividedinto 16×16 non-overlapping pixel blocks 32 and a code for tamperdetection and localization is generated. This is done by computing aCyclic Redundancy Code, CRC-16, which is an error error-checking code,for each of the blocks 32 using a computational function. The computedCRC bits form the code for tamper detection and localization informationwhich is then embedded into the digital medical image 20 using the samereversible watermarking process. Computing the CRC is preferred becauseit is computationally less intensive as compared to hash functions. Thisconsideration is important because a large number of CRC codes may haveto be generated, depending on the size of the image. Computational timebecome a crucial issue when watermarking medical images of volumescontaining multiple slices of DICOM images. In the tamper detection andlocalization function, the standard CRC-16-CCITT polynomial is usedtogether with a block size of 16×16 pixels. These parameters areselected based on the tradeoff between the area of detection, strengthof detection and the capacity to embed the tamper localizationinformation.

Using the same watermarking embedding algorithm, a CRC computed for aparticular block 321 is embedded back into that particular block 321, asshown by the arrows 4. In the event that the 16 bits of the CRC computedfor a first block 321 cannot be embedded into the first block 321, theremaining bits will be carried over to a second block 322 to be embeddedprior to embedding of the CRC of the second block 322, as indicated bythe arrow 7. If the remaining CRC bits of the first block 321 and theCRC of the second block 322 can all be embedded into the second block322, only the CRC of a third block 323 will be embedded into the thirdblock 323 itself. This method is preferred to simply concatenating theCRC as a string spanning all the blocks 32 because the latter willresult in a failure to retrieve the CRC of each block 32 when any of theembedded CRC bits is altered.

FIG. 3 illustrates a layer concept used in the reversible watermarkingprocess of the method. Two layers 25, 35 of watermarking are performedfor each digital medical image 10. In the first layer 25, data or sourceinformation such as metadata of the image is first embedded, followed byembedding of the digital envelope (DE), as described above. In thesecond layer 35, the generated code for tamper detection information isembedded as described above. The first layer 25 thus stores informationrelated to the source and information used to check the integrity of thedigital medical image 10 and message. The second layer 35 is designatedfor the tamper detection and localization function. Because of thereversible nature of the watermarking process, it is thus possible towatermark in the two layers 25, 35 and subsequently retrieve informationfrom the first layer 25 by removing the second layer 35 completely. Thisgreatly increases the amount of data that can be embedded in one digitalmedical image 10.

As shown in FIG. 9, when reviewing a transmitted digital medical imagethat has been prepared by the two layers 25, 35 of watermarking asdescribed above, the code for tamper detection and localization isretrieved from the digital medical image 92 by again dividing thedigital medical image into the same 16×16 blocks and extracting theembedded CRC using the steps described above for data extraction. Arestored image is obtained by reversing the watermarking processes 94,i.e., remove both layers 25, 35 of watermarking. A code is generatedfrom the restored image using the same computational function thatgenerated the code for tamper detection and localization 96. The codegenerated from the restored image thus comprises the CRC of each 16×16pixel block of the restored image. By comparing the retrieved code fortamper detection and localization with the code generated from therestored image 98, if both CRCs for a same block of pixels do not match,that block will be identified as having been tampered with, henceachieving tamper localization concurrently with tamper detection. Usingthe tamper detection and localization function, it is thus possible todetermine if and exactly where modifications have been made to an image.

Sample medical images in DICOM format were used to test the method. Theimage types used were those from Magnetic Resonance Imaging (MRI),Computed Tomography (CT), Ultrasound (US) and X-Ray Angiography (XA).Four important performance metrics were studied:

-   -   1. Embedding capacity: A measure of embedding capacity is        necessary to ensure that sufficient authentication information        can be embedded into the image.    -   2. Imperceptibility: This is to test the quality of the medical        images in terms of the invisibility of the watermark.    -   3. Run time: The time taken for the watermarking and        dewatermarking process of an image should be assessed to ensure        that it does not slow down the hospital's information system.    -   4. Robustness against tampering: This measure addresses the        effectiveness of the tamper detection and localization function        for detecting and locating alterations of pixels.        Embedding Capacity

Each sample image was embedded to its maximum capacity. ThePeak-Signal-to-Noise-Ratio (PSNR) and Mean-Squared-Error (MSE) werecalculated by comparing each original image with its watermarked image.Four sample images from different modalities and of different imagesizes were selected for the test. The DICOM test images were obtainedfrom third party software, in this instance OsiriX Image NavigationSoftware. None of the images used in the review had a maximum pixelvalue greater than 65527 which is a requirement for the watermarkingprocess to handle overflow and underflow. Table 1 below summarizes theperformance results.

TABLE 1 Bits Maximum Amount Run Image Image Per of Data That Can PSNRTime Type Size Pixel Be Embedded (Bits) MSE (dB) (s) MRI 512 × 512 1674190 21.5 34.8 5.48 XA 1024 × 8 581524 20.7 35.0 20.7 1024 US 480 × 64016 79865 21.9 34.7 6.36 CT 512 × 512 8 127078 21.9 34.7 6.00The number of bits that can be embedded for the four test images rangedfrom 74190 to 581524 bits. For a larger image size, the maximum numberof bits that can be embedded increases. For example, 581524 bits ofinformation can be embedded into an XA image, which has the largestimage size of 1024×1024 pixels. This was the largest embedding capacityof all four test cases. This is expected as more pixels are availablefor the hiding of information bits using the reversible watermarkingprocess. Although the MR and CT image were of the same size, there was adifference in embedding capacity. This is mainly because watermarkingprocess is dependent on the pixel correlation of the image. Highercorrelation (i.e. high similarity between pixel values) will result inhigher embedding capacity.Imperceptibility

The PSNR calculated for all images ranged between 34 and 35 dB. FIG. 4shows that images embedded at maximum capacity, i.e., FIGS. 4 (b), (d)and (f) are visually indistinguishable from the corresponding originalimages, i.e. FIGS. 4 (a), (c) and (e). It should be noted that a higherPSNR might not necessary translate to a better image quality. Forexample, a small distortion in a region of interest might still resultin a high PSNR but will have a significant impact on diagnosis results.Hence, it is important that original images can always be restored, asprovided for in the method by using the reversible watermarking process.

Run Time

Time taken for watermarking and dewatermarking is an important factor toconsider for practical use in any hospital system. It should not slowdown the hospital's information system. The results showed that the timetaken to prepare the test images and subsequently review them was anaverage of 9 seconds.

Robustness Against Tampering

In order to demonstrate the tamper detection and localization functionin detecting forgery, counterfeited images were created by manuallymodifying the pixel values in the watermarked images using an imageprocessing software, in this instance ImageJ. FIGS. 5 (b) and (e) aretwo samples of counterfeited images while FIGS. 5( c) and (f) show thecorresponding images with tampered regions being localized by the tamperdetection and localization function. The localized tampered blocks areshown in shaded boxes as circled 52, 54 in FIGS. 5( c) and (f).

The watermarked images of FIGS. 4 (b) and (d) were also put through asystematic tampering test. The tampering test included tampering asingle pixel, tampering a single block of size 8×8 bits and tampering aspread of 8×8 bit sized blocks. FIGS. 6 and 7 show representativeresults obtained using the tamper detection and localization functionfor an XA image and a CT image respectively. Tamper locations areindicated by circling 66, 77. The results show that the tamper detectionand localization function was able to achieve a 100% detection andlocalization rate, down to one pixel of tampering.

Whilst there has been described in the foregoing description preferredembodiments of the present invention, it will be understood by thoseskilled in the technology concerned that many variations ormodifications in details of design or construction may be made withoutdeparting from the present invention.

The invention claimed is:
 1. A method of preparing a digital medicalimage for secure transmission, the method comprising: embedding metadatainto the digital medical image using a reversible watermarking process;generating a code for tamper detection and localization from the digitalmedical image using a computational function; and embedding the code fortamper detection and localization into the digital medical image usingthe reversible watermarking process, wherein embedding the metadatausing the reversible watermark process comprises dividing the digitalmedical image into non-overlapping pixel blocks, generating a randomlocation signal designating one pixel of each non-overlapping pixelblock as an estimator pixel, and embedding the metadata into one or moreof the non-overlapping blocks as required.
 2. A method of reviewing adigital medical image prepared by the method of claim 1, the method ofreviewing comprising: retrieving the code for tamper detection andlocalization from the digital medical image; reversing the watermarkingprocesses to obtain a restored image; generating a code from therestored image using the computational function; and comparing theretrieved code for tamper detection and localization with the codegenerated from the restored image to detect and locate tampering.
 3. Themethod of claim 2, wherein generating the code from the restored imagecomprises dividing the restored image into non-overlapping pixel blocksand computing a cyclic redundancy code for each non-overlapping pixelblock of the restored image.
 4. The method of claim 1, furthercomprising encrypting the random location signal using public keycryptography.
 5. The method of claim 4, further comprising embedding adigital envelope into the digital medical image after embedding themetadata, the digital envelope comprising a concatenation of a bitstream of the encrypted random location signal, a cyclic redundancy codecomputed for the random location signal and a hash of the digitalmedical image.
 6. The method of claim 1, wherein generating the code fortamper detection and localization from the digital medical imagecomprises dividing the digital medical image into non-overlapping pixelblocks and computing a cyclic redundancy code for each non-overlappingpixel block.
 7. The method of claim 6, wherein embedding the code fortamper detection and localization using the reversible watermark processcomprises embedding each cyclic redundancy code into the non-overlappingpixel block for which the cyclic redundancy code was computed.
 8. Adigital medical image prepared for secure transmission using the methodof claim 1 or any one of claims 4 to
 7. 9. A method of securelytransmitting a digital medical image, the method comprising: preparingthe digital medical image using a method of: embedding metadata into thedigital medical image using a reversible watermarking process;generating a code for tamper detection and localization from the digitalmedical image using a computational function; and embedding the code fortamper detection and localization into the digital medical image usingthe reversible watermarking process, wherein embedding the metadatausing the reversible watermark process comprises dividing the digitalmedical image into non-overlapping pixel blocks, generating a randomlocation signal designating one pixel of each non-overlapping pixelblock as an estimator pixel, and embedding the metadata into one or moreof the non-overlapping blocks as required; transmitting the prepareddigital medical image; and reviewing the prepared digital medical imageusing a method of: retrieving the code for tamper detection andlocalization from the digital medical image; reversing the watermarkingprocesses to obtain a restored image; generating a code from therestored image using the computational function; and comparing theretrieved code for tamper detection and localization with the codegenerated from the restored image to detect and locate tampering.